Method of electropolishing medical implants

ABSTRACT

An electropolishing apparatus and method are provided for polishing stents and other medical implants. The apparatus includes a motor that rotates a roller. The roller continuously rotates the medical implant to be electropolished. One of the advantages of the apparatus and method is that marks generated around the electrical contact between the anode and the medical implant are minimized. In addition, the medical implant is polished more evenly than conventional electropolishing systems.

This application is divisional of U.S. patent application Ser. No.11/803,103, filed May 11, 2007, which is a continuation of U.S. patentapplication Ser. No. 10/712,420, filed Nov. 12, 2003, now U.S. Pat. No.7,252,746, both of which are hereby incorporated by reference herein.

BACKGROUND

The present invention relates generally to medical devices andparticularly to electropolishing medical implants.

Electropolishing is a widely used manufacturing process that provides asmooth surface finish to metallic parts. Typically, electropolishing isused after various forming operations, such as machining, punching,laser cutting, and electrodischarge cutting, to remove burrs, sharpedges and other rough features that are generated during the manufactureof metallic parts.

The basic concepts of electropolishing are well known to those in theart, and thus, only a brief summary is required here. Conventionalelectropolishing processes involve contacting a metallic part with ananode (i.e., a positively charged electrode) and spacing a cathode(i.e., a negatively charged electrode) away from the metallic part. Themetallic part, along with the anode and cathode, are then immersed in abath of electrolytic fluid. Next, a voltage is applied across the anodeand the cathode for a period of time. The effect of this is that metalfrom the metallic part is drawn away from the metallic part and is drawnto the cathode. (Although different in some respects, electropolishingmay be thought of conceptually as the opposite of electroplating.)Because burrs and sharp edges experience a higher current density thansmoother surfaces on the part, metal is removed from these areas at afaster rate than the rest of the metallic part. Thus, electropolishingprocesses leave a smooth surface finish in which the rough edges of themetallic parts are removed.

One application in which electropolishing is particularly useful is forfinishing endovascular stents and other medical implants. Medicalimplants require exceptionally smooth surfaces since any rough edges maycause tissue irritation during or after being implanted into a person'sbody. Some of the medical problems that may be encountered when roughedges are not properly removed from a medical implant includeinflammation, bleeding and/or scarring of the surrounding tissues. Inthe case of endovascular stents, such conditions can be particularlyharmful and dangerous. For example, one risk that may result from theuse of stents with rough edges is restenosis. Restenosis refers to there-narrowing of a vessel which sometimes occurs after balloonangioplasty procedures. Although restenosis may occur for a number ofreasons, tissue irritation and disturbance caused by rough edges on astent may be one cause of restenosis.

Various apparatuses for electropolishing stents have been tried.

One such apparatus involves wrapping a platinum wire (i.e., the anode)around the outer surface of the stent. The stent is then lowered into anelectrolytic both in a horizontal orientation (i.e., with the two endsof the stent being positioned at approximately the same height above thebottom of the bath). The cathode is formed as a single horizontal loopthat surrounds the stent (i.e., the loop defines a plane that isapproximately parallel to the bottom of the bath).

This apparatus suffers from several problems, however. One problem isthat marks are generated on the surface of the stent around the pointsof electrical contact between the platinum wire and the stent. This is acommon problem with electropolishing apparatuses and is not limited tothe particular electropolishing apparatus described here. This problemoccurs because the area of the stent located near the electrical contactbetween the wire and the stent experiences a higher current density thanthe rest of the stent. As a result, metal is drawn away from this areaof the stent at a particularly aggressive rate. In addition, the wireeffectively masks the portion of the stent which is in direct contactwith the stent, thus creating an area that experiences a minimal rate ofmetal removal. The result of this arrangement is that small grooves,pits and other marks are formed around the electrical contact in arandom pattern. Thus, the smooth surface finish which is desired acrossthe entire stent is not achieved due to the marking that occurs aroundthe electrical contact.

Another problem with this apparatus is that the metal removal rate isnot uniform across the entire stent. One problem is that the ends of thestent generally experience a higher metal removal rate than the center.This is caused in part by the closer proximity of the ends of the stentto the cathode. In contrast, the center region of the stent is locatedat or near the center of the cathode loop (i.e., farther away from thecathode loop itself). In addition, since the anode (i.e., the platinumwire) is wrapped around the outer surface of the stent, the innersurface of the stent experiences a lower metal removal rate than theoutside surface of the stent. In addition, because the anode (i.e., theplatinum wire) is wrapped around the outer surface of the stent, theinner surface of the stent may experience a lower metal removal ratethan the outside surface of the stent.

Uneven metal removal is a problem that many electropolishing apparatusessuffer from. In the case of stents, this problem can make manufacturingmore difficult and expensive since manufacturing tolerances need to beespecially tight in order to ensure proper performance of the stent.Thus, in electropolishing processes in which the metal removal ratevaries significantly across the stent, the percentage of manufacturingrejects may be higher, thereby raising costs.

Other typical electropolishing apparatuses include tree-like rackshaving a vertical center-stem and angled arms extending out from thecenter-stem. Stents are installed on each of the arms by sliding thestent over an arm so that the arm extends through the cylindrical cavityof the stent. Therefore, the tree-like rack functions as the anode bycontacting the inner surface of the stent. The cathode may be a cathodelike that previously described or may be a metal container that holdsthe electrolytic fluid.

This apparatus, however, suffers from problems that are similar to thosealready described. For example, marking around the electrical contactbetween the anode and the stent may also be a problem with thisapparatus. In addition, the diameter of the arm that extends through thecenter of the stent typically fills most of the center cavity of thestent. The reason for this is that the arms usually need to be builtstrong to avoid deforming the arms during loading, unloading and normalmanufacturing use. The problem with this design is that the largediameter of the arms prevents electrolytic fluid from circulating withinthe interior of the stent. As a result, the interior surfaces of thestent do not receive a consistent polish.

It is apparent to the inventor that an apparatus and method forelectropolishing medical implants is desired in which marking of themedical implant is minimized and metal removal is more consistent.Accordingly, a solution is described more fully below which solves theseand other problems.

SUMMARY

A method and apparatus are provided for electropolishing medicalimplants and stents. The method involves continuously rotating a stentwhile applying a voltage across an anode and a cathode. The electricalcontact between the anode and the cathode thereby continuously changes.This leads to a reduction in marks that are traditionally generatedaround the anode-stent contact. In addition, the apparatus provides amore uniform polishing of the stent. Additional details and advantagesare further described below.

BRIEF DESCRIPTION OF SEVERAL VIEWS OF THE DRAWINGS

The invention may be more fully understood by reading the followingdescription in conjunction with the drawing in which:

FIG. 1 is a perspective view of an electropolishing apparatus; and

FIG. 2 is a close-up perspective view of the electropolishing apparatus;and

FIG. 3 is a schematic view of the electropolishing apparatus, showing anelectrical circuit and an amp-hour meter.

DETAILED DESCRIPTION

Referring now to the drawings, an electropolishing apparatus 10 isprovided. The electropolishing apparatus 10 includes a frame 12 thatsupports the various components of the electropolishing apparatus 10.Although numerous types of frames may be used, the frame 12 which isshown is an open frame 12 made of high density polyethylene. Theelectropolishing apparatus 10 is designed to be immersed in anelectrolytic bath up to the top of the frame 12. The electrolytic fluidfreely passes through the open frame 12 and around the variouscomponents of the electropolishing apparatus 10 except the motor 18.Alternatively, other frames may be used, such as a closed frame thatalso defines a container for the electrolytic bath.

The electropolishing apparatus 10 further includes a roller 14 which isrotatably mounted within the frame 12. The roller 14 is mounted in theframe 12 at an angle between a vertical orientation and a horizontalorientation. The roller 14 is made from a non-conductive material, suchas high density polyethylene. Along the outer surface of the roller 14,longitudinal grooves 16 are provided which extend parallel to therotational axis of the roller 14.

An anode 20 is mounted within the frame 12 and is spaced away from theroller 14 and oriented parallel thereto. The anode 20 is preferably aplatinum wire 20 that is about 0.025 inch in diameter. Platinum ispreferred since platinum does not degrade during typicalelectropolishing processes. In order to provide sufficient stiffness, ahigh strength grade of platinum may be used, such as cold workedplatinum. The top end of the anode wire 20 is a free end 22, while thebottom end of the wire 20 is attached to the bottom of a swing arm 24.The wire 20 extends up through the swing arm 24 and is attached to apositive electrical charge, or other voltage potential. Preferably, theswing arm 24 is made of high density polyethylene. The swing arm 24 isattached to the top of the frame 12 by a hinge 26. Thus, the swing arm24 and the anode wire 20 may be rotated upward out of the frame 12around the hinge 26.

The cathode 28 includes three separate cathode loops 30. However, otherarrangements for the cathode are also possible, such as more or fewercathode loops 30, a solid plate, a wire mesh, or a metal container forthe electrolytic bath. In general, the cathode must be constructed toassure sufficient current flow from the anode 20 and stent 34 to thecathode 28. Preferably, the cathode loops 30 are made from the samematerial as the medical implant to be electropolished in order to avoidcontamination during electropolishing. Since the stent 34 describedbelow may be made from 316L stainless steel, the cathode loops 30 mayalso be made from 316L stainless steel. The stent 34 and cathode 28could also be made of other metal alloys, such as L605, MP35N, NiTi, orany other metal alloy that is commonly electropolished to improvesurface finishes. The cathode loops 30 each extend around the roller 14and the anode wire 20. The cathode 28 (represented by a bracketencompassing the three cathode loops 30) may or may not wrap all the wayaround the roller 14 and the stent 34. The cathode loops 30 are attachedto a support post 32 that extends up from the bottom of the frame 12.The cathode 28 is electrically connected to a negative electricalcharge, or other voltage potential.

A typical method of operating the electropolishing apparatus 10 follows.The electropolishing apparatus 10 is lowered into an electrolytic bathuntil the frame 12 is immersed in the electrolytic fluid withoutimmersing the motor 18. One example of the type of electrolytic fluidthat may be used is a mixture of sulfuric and phosphoric acids. However,any common electrolytic fluid may be used. The swing arm 24 is thenrotated upward so that the anode wire 20 rises out of the electrolyticbath. Next, a stent 34 is placed on the anode wire 20 by sliding thestent 34 down over the anode wire 20, with the wire 20 extending throughthe cylindrical cavity of the stent 34. The swing arm 24 is then rotatedback down into the electrolytic bath.

The electropolishing operation is started by operating the motor 18 atthe same time that a voltage is applied across the anode 20 and thecathode 28. The motor 18 rotates the roller 14, which in turn rotatesthe stent 34. The bottom end of the stent 34 rotates on the bottom ofthe swing arm 24, which provides a smooth rotational surface to avoidcatching the end of the stent 34 during rotation. The longitudinalgrooves 16 in the outer surface of the roller 14 assist rotation of thestent 34 by providing additional traction, or friction, between theroller 14 and the stent 34. Longitudinal grooves 16 oriented parallel tothe rotational axis of the roller 14 have been found to be better thanvarious types of helical grooves since helical grooves or other likefeatures may tend to drive the stent 34 either upward off the anode wire20 or downward into the swing arm 24. In addition to rotating the stent34, the roller 14 has the effect of pulling the stent 34 in thedirection that the roller 14 is rotating. As a result, the anode wire 20contacts the inside surface of the stent 34 along the side edge of theanode wire 20, thereby maintaining the position of the stent 34 on theroller 14 while permitting the stent 34 to roll. Preferably, the anodewire 20 extends through the entire length of the stent 34 so that theanode wire 20 contacts the stent 34 along a line across the full lengthof the stent 34.

Accordingly, an electrical contact is established between the anode wire20 and the stent 34. Since the stent 34 rotates during theelectropolishing operation, the electrical contact between the anodewire 20 and the stent 34 continuously changes. In a typicalelectropolishing operation of an endovascular stent 34, an electricalvoltage of about 2 to 6 volts is applied across the anode 20 and thecathode 28 until a satisfactory polish is achieved. In addition, thestent 34 is rotated about 35 revolutions per minute during theelectropolishing operation. A rotational speed between about 5revolutions per minute and 60 revolutions per minute may also provideimproved electropolishing results. In addition, the electrolytic bath isheated to about 60° Celsius during the electropolising operation. Asthose in the art now recognize, the current density applied to the stent34 causes metal to be removed from the stent 34. The charged metalparticles are then drawn through the electrolytic fluid to the cathodeloops 30. The removal of metal from the stent 34 results in a smoothpolishing effect, with any burrs and sharp edges being removed at afaster rate than the smooth surfaces of the stent 34.

In order to achieve more consistent polishing from part to part, thepolishing method is controlled through the use of an amp-hour meter 36,which measures the amount of electrons that pass through the circuit.Thus, the amp-hour meter 36 provides a more repeatable polish byadjusting the amount of time the stent 34 is polished if contact betweenthe anode 20 and the stent 34 becomes intermittent due to the changingcontact point. Therefore, the method may be controlled by establishing aspecific cumulative current flow instead of relying upon a set amount ofpolishing time (which may result in inconsistent polishing from onemedical implant to another).

The advantages of the electropolishing apparatus and method arenumerous.

One of the significant advantages is that the generation of marks aroundan electrical contact between the anode 20 and the stent 34 areeliminated and/or minimized. In conventional electropolishingapparatuses, these marks appear as an irregular pattern of small groovesor pits. However, by rotating the stent, and constantly changing theelectrical contact, the described apparatus 10 spreads the averagecurrent density more evenly around the stent 34, thereby preventing thehigh current density near the anode 20 from concentrating on a singlearea of the stent 34. Likewise, the area of the stent that is masked bythe anode 20 is constantly moved so that any particular area of thestent 34 experiences only a momentary masking effect as the electricalcontact moves around the stent 34. As a result, the electrical contactof the described apparatus 10 acts like an infinitely variableelectrical contact in contrast to conventional static or periodicelectrical contacts. Thus, compared to conventional electropolishingapparatuses and methods, stents 34 and other medical devices may bepolished with improved surface finishes by eliminating the marksassociated with the anode-stent electrical contact that are common withconventional systems.

Another significant advantage of the electropolishing apparatus 10 isthat the metal removal rate across the entire stent 34 is more uniformthan with conventional electropolishing systems. Uneven polishing is acommon cause of manufacturing rejects. As those in the art well know,the dimensions of a stent 34 must be closely monitored to ensure thatthe stent 34 will function in a reliable manner. One physical dimensionthat is closely monitored is the width of the struts of the stent 34. Insome prior art apparatuses, electropolishing has been so uneven that thewidth of the struts at the ends of the stent is significantly thinnerthan the width of the struts in the middle of the stent. The describedapparatus and method overcome this problem in part by continuouslyrotating the stent 34. As a result, the distance between the cathode andany given point on the stent 34 continuously changes. In effect, thedistance between the stent 34 and the cathode 28 is averaged for allpoints on the stent 34. Thus, the metal removal rate is equalized. Inaddition, the cathode 28 is made up of three cathode loops 30 that areequally spaced apart from each other. Thus, the distance between thecathode loops 30 and the stent 34 is further averaged and equalized.Therefore, it is apparent that the electropolishing apparatus 10 mayreduce expenses and improve the quality of stents 34 by reducingmanufacturing rejects and minimizing polishing variations.

The location and design of the anode 20 also offer several advantages.Since the anode 20 is made from a relatively small diameter wire 20, theanode wire 20 only fills a part of the cylindrical interior volume ofthe stent 34. This allows more electrolytic fluid into the center regionthan is possible with some prior art apparatuses that use largerdiameter stems that extend through the center of the stent. The greateramount of electrolytic fluid in the center region further facilitatesconsistent, even electropolishing. Although the wire diameter used inthe described apparatus is about 0.025 inch, a wire diameter as large as75% of the inner diameter of the stent may provide similar advantages.

The constant rotation of the stent 34 also has the effect of circulatingthe electrolytic fluid during electropolishing. This also facilitates amore consistent polishing effect. In particular, the stent structureitself (i.e., the struts and openings of the stent 34) stirs theelectrolytic fluid as the stent 34 rotates. Thus, electrolytic fluidcontinuously flows around and inside the stent 34. As mentioned above,the small diameter anode wire 20 permits a significant amount ofelectrolytic fluid into the center region as well. The benefit of thisdesign is that the electrolytic fluid in the center region is alsocirculated and mixed as the stent 34 rotates.

Another advantage of the anode wire 20 is that it contacts the stent 34on the inside surface of the stent 34. Thus, the electric current flowsbetween the inner surface of the stent 34 (i.e., where the anode 20contacts the stent 34) and the outer surface of the stent 34 (i.e., theclosest surface to the cathode 28). This provides a more consistentcurrent density across the entire stent 34, which again results in moreeven polishing.

The small diameter anode wire 20 has other advantages as well. Oneadvantage is that the cost of the electropolishing apparatus 10 may bereduced. As those in the art well know, platinum is an especiallyexpensive material. Thus, by making the anode 20 out of a small diameterwire 20, the amount of platinum is reduced and the cost of the apparatus10 is minimized. In contrast, other arrangements may be used that alsoconstantly rotate the stent 34 and continuously change the anode-stentcontact, such as using a large anode roller which contacts the outersurface of the stent or placing the stent in a rotating anode drum.However, these possible alternatives would greatly increase the amountof platinum, or other anode material, that would be needed, therebyincreasing the cost of the apparatus 10. Furthermore, a small anode isgenerally desired during electropolishing operations in order to obtainan accurate reading of the amount of metal removed. (The larger theanode, the less accurate the reading). Thus, the small diameter anodewire 20 has the advantage of enabling accurate metal removalmeasurements during the electropolishing operation compared to otheralternatives.

The orientation of the roller 14 and the design of the swing arm 24 alsooffer advantages. The angled roller 14 prevents the stent 34 fromwalking off the roller 14 as might happen if the roller 14 were orientedhorizontally. Moreover, the non-vertical orientation allows the stent 34to rest on top of the roller 14, thereby generating rotational frictionto roll the stent 34. In addition, the swing arm 24 may be rotatedupward and out of the electrolytic bath while leaving the roller 14 andthe motor 18 permanently mounted in place. This makes loading andunloading stents 34 quicker and easier. To unload a stent 34, the swingarm 24 may be rotated upward by hand without having to contact theelectrolytic bath. In fact, the swing arm 24 may be rotated 180° or moreuntil the free end 22 of the anode wire 20 is pointing downward and awayfrom the frame 12 of the electropolishing apparatus. The polished stent34 will then slide off the anode wire 20 by itself and may be allowed todrop into a collection bin. To load a new stent 34 that is to bepolished, the swing arm 24 is simply rotated so that the anode wire 20is located above the electrolytic bath with the free end 22 of the anodewire 20 pointing upward. The stent 34 is then mounted on the anode wire20 by sliding the stent 34 down the wire 20. The swing arm 24 may thenbe rotated back down into the electrolytic bath.

Accordingly, it is now apparent that there are many advantages of theinvention provided herein. In addition to the many advantages that havebeen described, it is possible that there are other advantages that arenot currently recognized but which may become apparent at a later time.

While a preferred embodiment of the invention has been described, itshould be understood that the invention is not so limited, andmodifications may be made without departing from the invention. Thescope of the invention is defined by the appended claims, and alldevices that come within the meaning of the claims, either literally orby equivalence, are intended to be embraced therein.

1. A method of electro-polishing a medical implant, comprising:establishing a predetermined cumulative current flow to be used tocontrol an amount of time said medical implant is electro-polished;immersing said medical implant, an anode and a cathode in anelectrolytic bath; contacting a surface of said medical implant withsaid anode, thereby forming an electrical contact; continuously movingsaid electrical contact between said medical implant and said anode insaid electrolytic bath, thereby continuously changing said electricalcontact; applying a voltage across said anode and said cathode,measuring an actual cumulative current flow between said anode and saidcathode; electro-polishing said medical implant until said actualcumulative current flow reaches said predetermined cumulative currentflow; and wherein said medical implant is electro-polished while beingimmersed in said electrolytic bath, the generation of marks on saidmedical implant at said electrical contact thereby being minimized. 2.The method according to claim 1, wherein said medical implant is rotatedbetween about 5 revolutions per minute and 60 revolutions per minute. 3.The method according to claim 2, wherein said medical implant is rotatedabout 35 revolutions per minute.
 4. The method according to claim 1,wherein said anode contacts an inner surface of said medical implant. 5.The method according to claim 1, wherein said anode extends along anentire length of said medical implant.
 6. The method according to claim1, wherein said medical implant is continuously rotated.
 7. The methodaccording to claim 1, wherein said anode is a wire extendinglongitudinally through a cylindrical cavity of said medical implant,said anode contacting an inner surface of said medical implant along aside surface of said wire.
 8. The method according to claim 7, whereinsaid anode has a diameter 75% or less than an inner diameter of saidcylindrical cavity.
 9. The method according to claim 1, wherein saidanode is made from platinum and said cathode is made from a samematerial as said medical implant.
 10. The method according to claim 1,wherein said anode is oriented at an angle with respect to said cathode.11. The method according to claim 1, wherein said anode is oriented atan angle between a horizontal orientation and a vertical orientation;and said anode is attached to a swing arm, said swing arm adapted tolift said anode and medical implant out of said electrolytic bath. 12.The method according to claim 1, wherein said anode contacts an innersurface of said medical implant and said medical implant is continuouslyrotated.
 13. The method according to claim 12, wherein said anode is awire extending longitudinally through a cylindrical cavity of saidmedical implant, said anode contacting an inner surface of said medicalimplant along a side surface of said wire, and said medical implant isrotated between about 5 revolutions per minute and 60 revolutions perminute.
 14. The method according to claim 13, wherein said anode has adiameter 75% or less than an inner diameter of said cylindrical cavityand said anode is made from platinum and said cathode is made from asame material as said medical implant.
 15. The method according to claim14, wherein said anode is oriented at an angle with respect to saidcathode and said anode extends along an entire length of said medicalimplant.
 16. The method according to claim 1, wherein medical implant iscontinuously rotated and said medical implant is rotated between about 5revolutions per minute and 60 revolutions per minute.
 17. The methodaccording to claim 16, wherein said anode is made from platinum and saidcathode is made from a same material as said medical implant.
 18. Themethod according to claim 17, wherein said anode is oriented at an anglewith respect to said cathode.
 19. The method according to claim 18,wherein said medical implant is rotated about 35 revolutions per minute.